Aluminum is a highly reactive metal that oxidizes when exposed to air. which of these would be a good first step in balancing this reaction?

It depends on which oxide is produced. The most common result is Al2O3.

So the first step is to find the oxide produced. Having done that your equation becomes.

Al + O2 ==> Al2O3 You have to pick a Common Multiple for O3 and O2. That would be your next set. In other words you want something that 3 and 2 will divide into evenly. That would be 6 The 3 on O3 divides into 6 evenly as 2

Al + O2 ===> 2Al2O3

Turn your attention to O2 on the left. 2 goes into 6 evenly. The answer is 3
Al + 3O2 > 2Al2O3

That leaves Al. The right side has 2 * 2 infront and behind the Aluminum That's 4
The balanced equation is
4Al + 3O2 ===> 2Al2O3

I have given you the entire procedure for balancing an equation. What you do first is a decision made by the person teaching the course.